1. Field of the Invention
The present invention relates to a method for estimating a remaining lifetime of a capacitor and the like, a temperature detecting structure suitable for the method, and various electronic equipment such as a power supply unit having such a capacitor.
2. Description of the Background Art
In electronic equipment of this kind, for example, a power supply unit, a lifetime of an electrolytic capacitor having the shortest lifetime among circuit components used for the power supply unit is estimated as a lifetime of the power supply unit.
It is known that the lifetime of the electrolytic capacitor can be calculated by the following arithmetic expression based on the Arrhenius law which is a law of temperature-lifetime showing a relation between a temperature and a lifetime:Lx=Lo×2(To−tx)/10×k
Lx: estimated lifetime at the time of actual use (time)
Lo: warranty lifetime (time) at a maximum using temperature
To: maximum using temperature (° C.)
tx: actual using temperature (° C.)
k: lifetime coefficient
Various equations for the lifetime coefficient k are proposed by capacitor makers, and the coefficient is a conversion coefficient obtained from applied voltage, ripple current, ambient temperature or the like.
Various estimating methods of lifetime based on such arithmetic expressions are conventionally proposed.
However, in the conventional technique, estimated lifetime is calculated using the arithmetic expression based on the Arrhenius law, actual operating time is subtracted from this estimated lifetime, and a result of this subtraction reaches a predetermined threshold value, it is judged that it is at the end of the lifetime, and an alarm is given. Therefore, the conventional technique can not inform a lifetime at the time of actual use.
Generally, exchange of electronic equipment used for various production equipment is incorporated in an annual schedule so that producing operation is not interfered. Therefore, the conventional technique in which an alarm informing that the electronic equipment is at the end of the lifetime is output or indicated suddenly one day can not be used easily. If the alarm informing that the equipment is at the end of the lifetime is output or indicated on a day other than a periodical maintenance term, there is a problem that the line must be stopped for exchanging the equipment.
Therefore, it is desired to estimate a remaining lifetime at the time of actual use, for example, to estimate remaining time during which the equipment can be operated.
Further, in order to precisely estimate a lifetime based on the Arrhenius law, it is necessary to precisely detect an actual using temperature (tx) of an electrolytic capacitor as apparent from the above arithmetic expression. A temperature sensor is provided for detecting the actual using temperature of the electrolytic capacitor, but since the detection is affected by convection of ambient air, it is not easy to precisely detect the temperature.
Especially in a power supply unit, a primary circuit and a secondary circuit are electrically insulated from each other by a transformer. The electrolytic capacitor whose lifetime is to be estimated is the primary circuit, but an arithmetic circuit such as a low-voltage operated microcomputer which calculates the lifetime based on a temperature detected by the temperature sensor is the secondary circuit. Therefore, it is more difficult to precisely detect a temperature of the electrolytic capacitor while keeping the insulation between the electrolytic capacitor and the temperature sensor.